How do you treat nouns connected by "and", "or", "nor", "who", "which", "that"?

1.Nouns connected by "and" -> treat as plural
2.Nouns connected by "or" or "nor" -> make the verb agree with the part of the subject nearer to the verb.
3."who", "which", "that" -> take verbs that agree with their antecedents.

How do you treat "the number" vs. "a number" plural or singular?

1."the number" -singular
2."a number" - plural

Fix:

"Lost Cities" describe the discoveries of many ancient civilizations.

"Lost Cities" [describes] the discoveries of many ancient civilizations.

Fix:

Statistics is among the most difficult courses in our program.

No change:

Statistics [is] among the most difficult courses in our program.

Fix:

Carmen is the only one of the applicants who have the ability to step into this position.

Carmen is the only one of the \applicants who\ [has] the ability to step into this position.

Fix:

Take a suit that travel well.

Take a suit that [travels] well.

Fix:

A tent and a sleeping bag is the required equipment.

A tent and a sleeping bag [are] the required equipment.

Fix:

At the back of the room is a small aquarium and an enormous terrarium.

At the back of the room [are] a small aquarium and an enormous terrarium.